We consider the ground state of a single spin-down impurity atom interacting attractively with a spin-polarized atomic Fermi gas. By constructing variational wave functions for polarons, molecules and trimers, we perform a detailed study of the quantum phase transitions between each of these bound states as a function of mass ratio $r=m_\uparrow/m_\downarrow$ and interaction strength. We find that Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing is mostly superceded by the formation of a $p$-wave trimer, which can be viewed as a FFLO molecule that has bound an additional majority atom. For sufficiently large $r$, we find that these transitions lie outside the region of superfluid-normal phase separation in spin-imbalanced Fermi gases and should thus be observable in experiment, unlike the well-studied equal-mass case. Reference: arXiv:1002.0101