Fix benchmarks

[mlykotom]

• Fix app suffix for benchmarks
• enable R8 for benchmarks
• add release build type to catalog, so that studio doesn't complain with variant conflicts (could/should we make it a composite build?)
• add helper ui automator function to check for number of children
• improve waiting for content
• select random authors

[mlykotom]

Ok, fixed the randomness and testing if author is already checked.

I regenerated the BLP profiles after the fixes with interesting numbers:

BEFORE

StartupBenchmark_startupBaselineProfile
   timeToFullDisplayMs   min 468.0,   median 516.8,   max 601.8
timeToInitialDisplayMs   min 244.7,   median 256.5,   max 271.8

AFTER

StartupBenchmark_startupBaselineProfile
   timeToFullDisplayMs   min 267.5,   median 298.2,   max 593.0
timeToInitialDisplayMs   min 179.7,   median 187.4,   max 353.3

[dturner]

LGTM with minor suggestions. Please merge once they're addressed.