fix: Remove placeholder on focus (#786)

## Description

<!-- Provide a concise and descriptive summary of the changes
implemented in this PR. -->

### Introduces a breaking change?

- [ ] Yes
- [x] No

### Type of change

- [ ] Bug fix (change which fixes an issue)
- [ ] New feature (change which adds functionality)
- [ ] Documentation update (improves or adds clarity to existing
documentation)
- [x] Other (chores, tests, code style improvements etc.)

### Tested on

- [x] iOS
- [ ] Android

### Testing instructions

Run llm demo apps and click on text box. The placeholder text like `Your
message` should disappear even if you do not write anything.

### Screenshots

<!-- Add screenshots here, if applicable -->

### Related issues

<!-- Link related issues here using #issue-number -->

Closes #782

- [x] I have performed a self-review of my code
- [ ] I have commented my code, particularly in hard-to-understand areas
- [ ] I have updated the documentation accordingly
- [x] My changes generate no new warnings

### Additional notes

<!-- Include any additional information, assumptions, or context that
reviewers might need to understand this PR. -->

Author: msluszniak

apps/llm/app/llm/index.tsx

@@ -97,7 +97,7 @@ function LLMScreen() {
                   ? ColorPalette.blueDark
                   : ColorPalette.blueLight,
               }}
-              placeholder="Your message"
+              placeholder={isTextInputFocused ? '' : 'Your message'}
               placeholderTextColor={'#C1C6E5'}
               multiline={true}
               ref={textInputRef}

apps/llm/app/llm_structured_output/index.tsx

@@ -179,7 +179,11 @@ function LLMScreen() {
                   ? ColorPalette.blueDark
                   : ColorPalette.blueLight,
               }}
-              placeholder="Your message e.g. I'm John. Is this product damaged? I can give you $100 for this."
+              placeholder={
+                isTextInputFocused
+                  ? ''
+                  : "Your message e.g. I'm John. Is this product damaged? I can give you $100 for this."
+              }
               placeholderTextColor={'#C1C6E5'}
               multiline={true}
               ref={textInputRef}

apps/llm/app/llm_tool_calling/index.tsx

@@ -146,7 +146,7 @@ function LLMToolCallingScreen() {
                   ? ColorPalette.blueDark
                   : ColorPalette.blueLight,
               }}
-              placeholder="Your message"
+              placeholder={isTextInputFocused ? '' : 'Your message'}
               placeholderTextColor={'#C1C6E5'}
               multiline={true}
               ref={textInputRef}