Optimize start info proto memory use

The proto reading and writing can allocate a lot of objects, re-use when possible.

NO_IFTTT=Not picking up labels correctly, will investigate separately.

Test: atest ApplicationStartInfoTest (testApplicationStartInfo covers persist and restore which leverages proto read/write)
Bug: 384539178
Flag: EXEMPT - small bugfix
Change-Id: I89b840b629a0b28deecf0e7078999a96e0c9cf9e