[Thread] rename package name for Thread platform flags

(This is a cherry-pick of aosp/2968526 for resolving merge conflicts)

There are two Thread flag files in frameworks/base and
packages/modules/Connectivity/thread. Both are currently using the same
"com.android.net.thread.flags" package name for their flags. While all
flags needs to be built in a aconfig_declarations target, two aconfig_declarations
can't have the same package name:

```
FAILED: out/soong/build.aosp_arm.ninja
cd "$(dirname "out/host/linux-x86/bin/soong_build")" && BUILDER="$PWD/$(basename "out/host/linux-x86/bin/soong_build")" && cd / && env -i  "$BUILDER"     --top "$TOP"     --soong_out "out/soong"     --out "out"     --soong_variables out/soong/soong.aosp_arm.variables -o out/soong/b
uild.aosp_arm.ninja --globListDir aosp_arm --globFile out/soong/globs-aosp_arm.ninja -l out/.module_paths/Android.bp.list --available_env out/soong/soong.environment.available --used_env out/soong/soong.environment.used.aosp_arm.build Android.bp
2 aconfig_declarations found for package com.android.net.thread.flags
```

To resolve this issue, this commit renames the platform flags to package
name "android.net.thread.platform.flags".

This commit is required by ag/26286387

Bug: 325886480
Merged-In: Icacf88f7c0aa499b4b29f4072ae58840f0b1761f
Change-Id: Icacf88f7c0aa499b4b29f4072ae58840f0b1761f