Allow purpose and valid-purpose child tags in the manifest.

This change is required to avoid AAPT2 from failing when <purpose> and
<valid-purpose> child tags are used in the manifest.

Bug: 419560908
Test: used tags in platform manifest and ran m -j99
Flag: android.permission.flags.purpose_declaration_enabled
Change-Id: Iaf6f67159b2194d1dbdebb9c1d2dd90b75efb8f9